Method of manufacturing a bimetallic grinding wheel

ABSTRACT

The method comprises the step of arranging at the periphery of a shell (10) capable of withstanding a temperature of at least 400° C. inserts (16) of highly wear-resistant material, preheating the shell (10) and the inserts (16) in an oven, removing rapidly the shell (10) together with the inserts (16) from the oven, placing them on a centrifugal casting machine which is set in rotation, pouring ductile cast iron (18) and demoulding after cooling.

BACKGROUND OF THE INVENTION

The present invention relates to a method of manufacturing a bimetallicgrinding wheel of generally frustoconical or cylindrical shape,comprising a cast support made of machinable ductile cast iron in theouter surface of which are embedded, longitudinally, in the direction ofthe generatrix, wear inserts made of a material having a high wearresistance, the said inserts being retained in the support by mechanicalbonding. The invention also relates to a grinding wheel obtained by theimplementation of this method.

Grinding wheels consisting of an outer wear layer which is supported bya more ductile core are known. These wheels are produced by successivecasting of the wear layer and of the supporting core in order to form ametallurgical bond between the two materials. Given that this method ofmanufacture can be implemented only for a few pairs of compatiblematerials for a metallurgical bond, the choice of materials, both forthe wear layer and for the supporting core, is limited. To produce themetallurgical bond furthermore requires perfect synchronization of thecasting operations and good control of the casting temperature.

Document EP 0,476,496 A1 recommends the technique of using inserts forthe manufacture of grinding wheels by the implementation of a method asdescribed in the preamble.

According to this document, the inserts are cast separately in firstmoulds and subsequently placed in a sand mould into which the supportingcore is poured in order to form a mechanical bond between the twomaterials. Contrary to the metallurgical bond, the mechanical bond doesnot involve constraints from the choice-of-materials standpoint, so thatit is possible to choose harder alloys for the inserts, for examplehigh-alloy chromium cast irons or any other highly wear-resistantmaterial, and more suitable alloys for the core, such as spheroidalgraphite cast iron.

This technique also enables the casting capacity to be doubled, giventhat the inserts are cast separately and may be stored for the purposeof the second casting.

However, this technique often comes up against other difficulties. Whenthe second casting is carried out into the sand mould on cold inserts,these run the risk of cracking induced by heat shock. In order to avoidthis risk, or at least to reduce it, it is necessary to preheat theinserts to a temperature of the order of 400° to 500° C. Now, thispreheating is not easy to perform given that it must entail bringingbulky heating equipment close to the sand mould and, in addition, thenecessary temperature to be reached generally causes the binder used forthe mould sand to be destroyed.

It is also necessary to fix the inserts temporarily in the sand mould inorder to prevent them from moving during casting. For this purpose, adestructible temporary support may be provided or, they may be simplywedged against each other in order to jam them by a vault effect.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a novel method ofmanufacturing bimetallic grinding wheels which enables the drawbacksassociated with the known method to be eliminated or at least reduced,while still keeping their advantages.

In order to achieve this objective, the invention provides a method ofthe kind described in the preamble, which is characterized in that amould is used which consists of a transportable shell made of metal orany other material capable of withstanding a temperature of at least400° C., in that the inserts are stood up at the periphery of the shell,in that the shell and the inserts are preheated in an oven, in that theshell together with the inserts are rapidly removed from the oven andplaced on a centrifugal casting machine which is set in rotation, inthat the ductile cast iron is poured and in that demoulding takes placeafter cooling.

The ductile cast iron is preferably a lamellar or nodular grey castiron, while the inserts are preferably made of chromium cast iron or ahighly wear-resistant material.

Before installing the inserts in the shell, it is provided with a thinrefractory coating.

The oven may be preheated to a temperature of the order of 100° to 500°C.

The centrifugal casting machine is rotationally driven at a speed of theorder of 100 to 600 revolutions per minute.

The wheel may be demoulded when the temperature has fallen to a value ofthe order of 500° C.

This method makes it possible to keep all the advantages obtained withthe insert technique, especially those which are due to the presence ofthe mechanical bond between the inserts. Furthermore, the use of atransportable shell as a mould eliminates the problems of preheating theinserts, given that this preheating may now be carried out in an oven inwhich the shell is placed together with the inserts installedbeforehand. This also enables the preheating temperature to becontrolled better.

Casting using centrifuging provides, moreover, an effect of naturalcomplementarity, beneficial to the insert technology. The centrifugalforce generated by the rotation of the shell automatically locks theinserts against the wall of the shell and thus contributes to betterfixing of the inserts and to a reduction in the risk of movement duringcasting. Furthermore, under the effect of the centrifugal force, thecast iron completely fills the spaces between the inserts in order toclamp them perfectly, without the risk of forming cavities due to thesolidification of the cast iron before all the interstices are filled.

BRIEF DESCRIPTION OF THE DRAWING

Other features and characteristics of the invention will emerge from thedetailed description of an advantageous embodiment, presented below, byway of illustration, with reference to the appended figure whichillustrates an axial section through a mould before a wheel producedaccording to the present invention is demoulded.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The mould shown in the figure is in the form of a transportable shell10, the volume of which corresponds to a grinding wheel, in this case anaxisymmetrical truncated cone. The shell 10 includes a closure lid 12with a central pouring hole 14.

The first step consists in coating the inside of the shell with a thinrefractory layer in order to protect the shell and to make the mouldingeasier. Next, inserts 16, cast beforehand in another mould, not shown,are arranged in the shell. These inserts 16 may be made of anywear-resistant material (eg. carbides, ceramics, etc.) or chromium castiron, of the kind described in document EP 0 476 496, in which case theymay be wedged against each other and jammed by a vault effect.

Next, the shell 10, together with the inserts 16, is placed in an ovenpreheated to a temperature of 100° to 500° C., depending on the natureof the materials used, so as to heat up the inserts to a temperaturesufficient to prevent heat shock.

When the inserts are at the desired temperature, the shell 10 is rapidlyremoved from the oven and placed on the rotary platform of a centrifugalcasting machine known per se. This machine is driven at a speed whichmay vary between 100 and 600 revolutions per minute in order to rotatethe shell, together with the inserts, about its vertical axis (arrow A).During the rotation of the shell 10, the supporting material for thewheel, for example lamellar or nodular grey cast iron, is poured throughthe pouring hole 14. Under the effect of the centrifugal force, the castiron spreads out in the form of a frustoconical layer over the inserts16 and fills all the interstices between them. After this casting, theassembly is allowed to cool and demoulding takes place at a temperatureof the order of 500° C. The wheel thus produced includes an axialfrustoconical bore enabling it to be fixed, by shrink-fitting, to asupport hub after a finishing machining operation.

We claim:
 1. Method of manufacturing a bimetallic grinding wheel ofgenerally frustoconical or cylindrical shape, comprising a cast supportmade of machinable ductile cast iron in the outer surface of which areembedded, longitudinally, in the direction of the generatrix, wearinserts made of a highly wear-resistant material, the said inserts beingretained in the support by a mechanical bond, the method comprising thesteps of:providing a transportable shell (10) made of a material capableof withstanding a temperature of at least 400° C.; installing theinserts (16) at the periphery of the shell (10); preheating the shell(10) in an oven; rapidly removing the shell (10) from the oven, andplacing the shell (10) on a centrifugal casting machine; rotating thecentrifugal casting machine to rotate the shell (10); pouring ductilecast iron (18) into the rotating shell (10) to form the bimetallicgrinding wheel; and demoulding the bimetallic grinding wheel aftercooling.
 2. Method according to claim 1, characterized in that theductile cast iron is a lamellar grey cast iron.
 3. Method according toclaim 1, characterized in that the inserts are made of high-alloychromium cast iron.
 4. Method according to claim 1, characterized in thefurther step of:providing the shell with a thin refractory coatingbefore the inserts (16) are installed.
 5. Method according to claim 1,characterized in that the shell and the inserts are preheated in theoven at a temperature of the order of 100° to 500° C.
 6. Methodaccording to claim 1, characterized in that the centrifugal castingmachine is rotated at a speed of the order of 100 to 600 revolutions perminute.
 7. Method according to claim 1, characterized in that demouldingtakes place at a temperature of the order of 500° C.
 8. Method accordingto claim 1, characterized in that the ductile cast iron is a nodulargrey cast iron.